Tea Polyphenols EGCG and Theaflavin Inhibit the Activity of SARS-CoV-2 3CL-Protease In Vitro

COVID-19, a global pandemic, has caused over 750,000 deaths worldwide as of August 2020. A vaccine or remedy for SARS-CoV-2, the virus responsible for COVID-19, is necessary to slow down the spread and lethality of COVID-19. However, there is currently no effective treatment available against SARS-CoV-2. In this report, we demonstrated that EGCG and theaflavin, the main active ingredients of green tea and black tea, respectively, are potentially effective to inhibit SARS-CoV-2 activity. Coronaviruses require the 3CL-protease for the cleavage of its polyprotein to make individual proteins functional. EGCG and theaflavin showed inhibitory activity against the SARS-CoV-2 3CL-protease in a dose-dependent manner, and the half inhibitory concentration (IC50) was 7.58 μg/ml for EGCG and 8.44 μg/ml for theaflavin. In addition, we did not observe any cytotoxicity for either EGCG or theaflavin at the concentrations tested up to 40 μg/ml in HEK293T cells. These results suggest that upon further study, EGCG and theaflavin can be potentially useful to treat COVID-19.


Introduction
A new coronavirus disease  has become the most dangerous pandemic of this century causing over 20,000,000 reported infections and over 750,000 reported deaths as of August 2020. However, a vaccine is not yet available, and effective remedies to treat COVID-19 are still under development. erefore, the infections and deaths due to COVID-19 will continuously increase until an effective vaccine or treatment is available. For this reason, various approaches to slow down the spread of COVID-19 need to be identified and developed.
Virus-specific enzymes are most often the main targets of antiviral medicines. For example, the thymidine kinase enzyme of herpesvirus is the main target of herpesvirus therapies, and thymidine kinase inhibitors including acyclovir and ganciclovir were developed to treat herpesvirus diseases [1]. Similarly, coronavirus-specific enzymes should be potential targets to treat coronavirus diseases. e 3CLprotease is regarded as the main drug target for coronavirus diseases [2]. Cleavage of viral polyproteins by proteases is a vital step in the life cycle of coronaviruses, and several viral proteases encoded in the coronavirus RNA are required for the maturation of the viral proteins [3]. e 3CL-protease is responsible for the cleavage of viral polyproteins of coronavirus and absolutely required for replication of the virus [4][5][6]. In light of its importance, the 3CL-protease is a target for antivirals, and several antiviral candidates were identified using a 3CL-protease assay [7][8][9]. Recently, chemicals targeting the SARS-CoV-2 3CL-protease have been reported to be effective to slow down the replication of coronavirus in vivo [10].
Green tea is a popular beverage, and many reports demonstrated that green tea has health benefits including cancer prevention [11,12]. Epigallocatechin-3-gallate (EGCG) is the major ingredient in green tea and accounts for 50% to 80% of a brewed cup of green tea [11,13]. EGCG or green tea showed a wide range of antiviral activity against adenovirus, influenza virus, zika virus, herpesvirus, and hepatitis virus [14][15][16][17][18][19][20]. In addition, a previous report demonstrated that green tea has antiviral activity toward coronaviruses [21]. eaflavin is an active ingredient of black tea, a fermented or oxidized form of green tea [22]. eaflavin also showed antiviral activity against influenza virus, herpesvirus, rotavirus, and coronavirus [23][24][25].
Since previous studies suggest that the active ingredients of green tea or black tea are effective to inhibit coronavirus 3CL-protease, we examined whether EGCG and theaflavin showed inhibitory effects on SARS-CoV-2 3CL-protease. In this study, we chemically synthesized the gene encoding SARS-CoV-2 3CL-protease using the published SARS-CoV-2 genome sequence and then utilized the protease assay to evaluate the inhibitory effect of EGCG and theaflavin.

Protease Assay for SARS-CoV-2 3CL-Protease.
A FRETbased protease assay was used to measure 3CL-protease activity [27]. Dabcyl-KTSAVLQSGFRKME-Edans was chemically synthesized (Anygen, Gwangju, South Korea) and used for the SARS-CoV-2 3CL-protease substrate. e 3CL-protease activity was performed at 37°C using 3CLprotein and FRET peptide in the reaction buffer (20 mM Tris-HCl (pH 7.5), 200 mM NaCl, 5 mM EDTA, 5 mM DTT, and 1% DMSO) for 5 h. For the inhibition assay, the purified 3CL-protease was incubated with EGCG or theaflavin for 1 h before the addition of substrate. e fluorescence was measured at 528 nm with excitation at 360 nm using a Synergy HTX multimode microplate reader (Biotek, Winooski, VT). Protease activity was calculated as the difference between the activity with 3CL-protease and the activity without 3CL-protease at the indicated time.

Statistical Analysis.
e results of 3CL-protease activity and MTT were evaluated by a 2-tailed Student's t-test using Excel software (Microsoft, Redmond, WA, USA). A p value of 0.05 was considered significant. For the calculation of half inhibitory concentration (IC 50 ), the AAT Bioquest website program was used (https://www.aatbio.com/tools/ic50calculator-v1). e coefficient of drug interaction (CDI) was calculated to determine the drug interaction between two different drugs. CDI is defined by the following formula: CDI � AB/(AxB) [28].

Expression of SARS-CoV-2 3CL-Protease.
Recently, the nucleotide sequence of the SARS-CoV-2 genome was published [29]. We aligned the SARS-CoV-2 3CL-protease peptide sequence with MERS 3CL-protease and SARS 3CL-protease ( Figure 1(b)). Based on the amino acid sequence analysis, SARS-CoV-2 3CL-protease shares 96.08% identity with SARS 3CL-protease and 49.51% identity with MERS 3CL-protease (Figure 1(b)). Sequence alignment data indicate that SARS-CoV-2 3CL-protease is more homologous to SARS 3CLprotease than MERS 3CL-protease. We used the SARS-CoV-2 3CL-protease nucleotide sequence to synthesize the gene that encodes the 3CL-protease protein. In order to conduct the SARS-CoV-2 3CL-protease assay, we expressed a His-tagged 3CL-protease in bacteria by IPTG induction and purified the 3CL-protease by a His-tag affinity column (Figure 2(a)). SARS-CoV-2 3CL-protease is readily detected in the soluble fraction, and we purified the 3CL-protease for the 3CL-protease assay (Figure 2(a)). To identify potential inhibitors of 3CL-protease activity, we used the 3CL-protease assay. As expected, the 3CLprotease assay showed higher activity as the incubation time increased up to 5 h (Figure 2(b)).

e Inhibitory Effect of EGCG on SARS-CoV-2 3CL-Protease Activity.
We examined the effect that EGCG has on the 3CL-protease activity by testing various concentrations of EGCG (0, 1, 2, 5, 10, 20, and 40 μg/ml). We found that EGCG significantly inhibits 3CL-protease activity in a dosedependent manner (Figure 3(a)). We calculated the half inhibitory concentration (IC 50 ) of EGCG using the AAT Bioquest website program and calculated an IC50 for EGCG of 7.58 μg/ml (Figure 3(b)). ese results indicate that EGCG is an inhibitor of SARS-CoV-2 3CL-protease.
e results showed that theaflavin significantly inhibits 3CLprotease activity in a dose-dependent manner similar to EGCG (Figure 4(a)). e half inhibitory concentration (IC 50 ) of theaflavin was 8.44 μg/ml, slightly higher than that of EGCG (Figure 4(b)). ese results indicate that EGCG is a more effective inhibitor of SARS-CoV-2 3CL-protease activity than theaflavin.

Additive effect of EGCG and eaflavin.
Because EGCG and theaflavin both inhibit SARS-CoV-2 3CL protease, we examined whether EGCG and theaflavin together may have an additive or a synergistic inhibitory effect on 3CL-protease activity. We incubated EGCG alone, theaflavin alone, and EGCG/theaflavin and measured the inhibitory effect on 3CL-protease. Using the observed protease activity, we determined the coefficient of drug interaction (CDI) and calculated a CDI for EGCG and theaflavin of 0.93, indicating that EGCG and theaflavin had an additive rather than a synergistic effect ( Figure 5).

Inhibitory Effect of EGCG Auto-Oxidation Products (EAOPs) on SARS-CoV-2 3CL
Protease. EGCG is susceptible to oxidation, and auto-oxidation of EGCG produces EGCG auto-oxidation products (EAOPs) in a time-dependent manner [26]. We examined whether EAOPs retain inhibitory activity on the 3CL protease. e color change indicates the production of EAOPs, and the inhibitory activity of EAOPs was not significantly changed up to 12 h ( Figure 6). Although the inhibitory activity was decreased at 24 h, EAOPs still retain a significant inhibitory effect on SARS-CoV-2 3CL-protease ( Figure 6).

Cytotoxicity of EGCG and eaflavin. Previous reports
showed that EGCG and theaflavin showed cytotoxicity on cultured cells [30,31]. erefore, we examined whether EGCG and theaflavin affected the cell viability of HEK293T cells. We incubated various concentrations of EGCG (0, 2.5, 5, 10, 20, and 40 μg/ml) with HEK293T cells for 24 h and found that EGCG and theaflavin did not affect the cell viability significantly at any of the concentrations tested (Figure 7). ese results suggest that EGCG and theaflavin can inhibit 3CL-protease without significant cellular cytotoxicity.

Discussion
e discovery for a COVID-19 treatment is urgent as COVID-19 spreads very rapidly, and infections and deaths are continuously rising all over the globe. SARS-CoV-2 3CLprotease is a promising target for COVID-19 treatments. Recently, a peptidomimetic chemical α-ketoamide was reported to inhibit the SARS-CoV-2 3CL-protease and reduce viral replication in cell culture [10]. However, further timeconsuming testing must occur to determine the safety and efficacy of these chemicals.
In this study, we aimed to determine whether EGCG and theaflavin, the major active components of green tea and black tea, have inhibitory activity against 3CL-protease. Green tea and black tea are consumed widely and regularly  throughout the world, and their health benefits have been universally lauded. We demonstrated that both EGCG and theaflavin inhibit 3CL-protease activity in a dose-dependent manner. We calculated the IC 50 of EGCG and theaflavin and found 7.58 μg/ml for EGCG and 8.44 μg/ml for EGCG (Figures 3 and 4). eaflavin inhibits SARS 3CL-protease with an IC 50 value of 7 μg/ml, and various tea extracts were assayed against SARS coronavirus 3CLprotease, with IC 50 values in the range of 25∼125 µg/ml [21]. In addition, black tea, an oxidized form of green tea, neutralizes bovine coronavirus infectivity at an EC 50 of 34.7 μg/ml [24]. e inhibitory activity of EGCG and theaflavin on SARS-CoV-2 3CL-protease is consistent with the inhibitory activity of SARS 3CL-protease observed in a previous study [21]. In addition, the purified single compounds (EGCG and theaflavin) show lower IC 50 than tea extracts, suggesting that EGCG and theaflavin are the main active ingredients that inhibit 3CL-protease.
One major question is whether effective levels of either EGCG or theaflavin can be reached in the body. When we calculated the IC 50 using the molar concentration of EGCG or theaflavin, the IC 50 is equal to 16.5 μM for EGCG and

EAOPs
Relative activity (%) Figure 6: EGCG auto-oxidation products (EAOPs) show the inhibitory effect on SARS-CoV-2 3CL protease. Auto-oxidation of EGCG was carried out to produce EAOPs for the indicated hours. EAOPs (10 μg/ml) were incubated with 3CL-protease, and the 3CLprotease activity was determined (top panel). "Con", mock control. e mean and standard deviation are shown. * p < 0.05; NS, not significant. Time-dependent color changes of EAOPs were shown (bottom panel).
Evidence-Based Complementary and Alternative Medicine 15.0 μM for theaflavin. However, the maximum blood concentration of EGCG is less than 1 μM and the maximum blood concentration of theaflavin is less than 0.1 μM [32][33][34]. e effect of EGCG or theaflavin, therefore, on coronavirus replication in the human body may be limited on their own. However, our data also showed that EGCG and theaflavin treatment together has an additive effect ( Figure 5). Green tea or black tea may provide various EGCG and theaflavin derivatives, and their additive effect may be effective to limit coronavirus replication. Moreover, EGCG can be oxidized in physiological conditions of body and converted into EAOPs.
Here, we showed that EAOPs retain inhibitory activity on the 3CL-protease, suggesting that EGCG can be effective for a long time ( Figure 6).
Here, we showed that EGCG and theaflavin, the active ingredients of green tea and black tea, are effective to inhibit the 3CL-protease in vitro. Because green tea and black tea contain a high percent of EGCG and theaflavin, it would be valuable to examine the effect of green tea and black tea on the spread of SARS-CoV-2 in vivo. In addition, further clinical trials will be required to reveal the effect of tea consumption on COVID-19 prognosis.

Data Availability
e original data that support the findings of this study are included in the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.

Authors' Contributions
Minsu Jang and Yea-In Park contributed equally to this work. Theaflavin (μg/ml) EGCG (μg/ml) Relative cell viability (%) Figure 7: Cytotoxicity of EGCG and theaflavin. EGCG and theaflavin did not have significant cytotoxicity up to 40 μg/ml. HEK293T cells were incubated with either EGCG or theaflavin for 24 h, and the MTT assay was performed to evaluate the cytotoxicity. 6 Evidence-Based Complementary and Alternative Medicine